I have this vague memory of a film which had haunted me as a kid. I remember watching it in the 90s, so it can't have been later than 99, but i feel like it was probably between the 80s and the 90s.

This is what I remember:

The opening scene faded into a baseball game, or kids playing baseball or catch or something along those lines.The movie revolved around our main character who was getting bullied by this one guy.The main character was at some stage running away from the bully, who was following him around trying to beat him up, the main character says "eat s**t" and the bully says something like "you will eat s**t" and then smushes the main character's head into dog poop that was on the ground.Much later on in the movie I remember the main character (and his friends?) having tied up the bully to a post in a barn/garage/basement (someplace dark and secluded) and threatened him with a gun, and maybe even shot him (?)

Here's the trailer. It seems to support the bully, baseball, and gun aspect of your description: http://www.youtube.com/watch?v=yVyO15J3sQo

Here's a synopsis:In this youthful drama, three 13 year olds come of age when they learn to stand up to a vicious local bully. Joey, Book and Mouth live for baseball. Joey is a superb pitcher and Book an excellent catcher while Mouth dreams of becoming Joey's agent. All three boys have less than perfect home lives, with their chief problem being that their parents are too often gone. Still, they have a good summer until Hayes, a nearly psycho bully begins tormenting them in increasingly violent ways.